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Old March 30th 11, 04:28 PM posted to rec.radio.amateur.antenna
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Default Helical-wound Monopoles

"Cecil Moore" wrote

Take your NEC helical model and adjust the frequency to
approximately double the resonant frequency and take a
look at the current distribution.


I have already done an illustration based on the currents in the NEC
comparison posted earlier, showing a helix and a linear monopole each about
6 degrees in aperture (link below).

This link shows that even though the length of wire used in the helix is
3.14 X the length used in the linear monopole, the current distribution
along their apertures essentially is the same, as will be the directivity
and radiation pattern of both versions.

This same equivalence would apply to the current distribution, directivity
and pattern of a linear, 1/4-WL monopole and a helically-wound monopole that
was 1/4-WL in aperture, but contained 1/2-WL of coiled wire.

http://i62.photobucket.com/albums/h8...le_Current.gif

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Old March 30th 11, 08:28 PM posted to rec.radio.amateur.antenna
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Posts: 572
Default Helical-wound Monopoles

On Mar 30, 10:28*am, "Richard Fry" wrote:
I have already done an illustration based on the currents in the NEC
comparison posted earlier, showing a helix and a linear monopole each about
6 degrees in aperture (link below).


What we have here is a failure to communicate. Please forget about
your previous posting. We are not talking about 6 deg. electrically
short helicals. We are talking about comparing an 180 degree
electrically long monopole to a 90 degree long RESONANT monopole.
Here's how to accomplish what we are talking about:

1. Wind a helical that is 90 degrees long, i.e. the feedpoint
impedance is R1+j0. That helical is 1/4WL long electrically and
resonant. It may be ~1/8WL (45 deg) long physically.
2. Now increase the frequency until the helical is 180 degrees long
electrically. At something like double the frequency, it will be 1/2WL
long electrically and the feedpoint impedance will be R2+j0 where
R2R1. It may be ~1/4WL long physically.

John said his 180 degree helical outperformed his resonant 90 degree
helical. His statement has nothing to do with electrically short
helical monopoles because they are resonant.

The current maximum for a 90 degree resonant helical will be at the
base feedpoint just as it is for a 90 degree stub. The current maximum
for a 180 degree helical will be halfway up the antenna just as it is
halfway up a 180 degree stub.
--
73, Cecil, w5dxp.com

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